1. Field of the Invention
The present invention relates to freeze drying apparatus, and more particularly, to shelf structures for utilization in freeze drying apparatus.
2. Description of the Prior Art
The method of freeze drying biological specimens and other materials such as medicine and food products by sublimation of ice in a vacuum has been known for over fifty years. It was not until shortly before World War II, however, that the true commercial potential of laboratory freeze dryers and the process of sublimation were recognized. Particularly, during World War II, substantial development was made in the equipment and techniques for the purposes of supplying medical products to the armed forces. Since that time, increased interest by food processors as well as pharmaceutical manufacturers has resulted in further development of freeze drying equipment. Thus, freeze drying has found application not only in the laboratory for various scientific purposes, but commercially as well.
Basically, the process of freeze drying involves the lowering of the temperature of a moisture-containing item or sample until it is in a completely solid state, i.e., until it is frozen. The sample is then maintained in the area of a very low absolute pressure or high vacuum and subjected to a controlled heat input. Application of the heat to the product at a controlled rate results in the water content of the frozen sample being sublimated (i.e., converted directly from a solid to a gas without passing through the liquid state). The gaseous water vapor is then effectively removed from the system by being refrozen onto a refrigerated condenser thereby protecting the vacuum pump oil from contamination by water vapor. The refrozen moisture can be removed from the condenser when the drying process is completed. The condenser can be located in the same chamber as the shelf assembly or in a separate condensation chamber. Representative examples of some prior art freeze drying apparatus are shown in U.S. Pat. No. 3,795,986--Sutherland et al., U.S. Pat. No. 3,950,963--Sutherland, U.S. Pat. No. 3,286,366--Seligman, U.S. Pat. No. 3,271,874--Oppenheimer and U.S. Pat. No. 4,173,078--Bird, et al.
In the types of freeze drying apparatus which are used for commercial purposes, such as freeze drying medicines, the material to be dried is usually placed in glass vials or containers supported on a shelf arrangement within the drying chamber. The vials typically have stoppers partially inserted in the open ends thereof, but the stoppers have slots which allow the moisture to escape from the interior of the vial during the freeze drying process. However, once the drying process has been completed, the vials must be closed before the drying chamber is open to prevent contamination when moisture-containing atmosphere enters the drying chamber. Consequently, various types of shelf arrangements have been provided which allow the shelves to be raised one against the other so that the vials are squeezed "accordion style" between the shelves driving the stoppers into the vials thereby sealing the vials. In other prior art freeze dryers, each shelf remains stationary but the pan or receptacle in which the vials are placed is positioned over an inflatable bladder. A plate large enough to cover all of the vials in one pan is placed on the tops of the stoppers. When the freeze drying sequence is completed, the bladder is inflated, thereby causing the pan to be moved vertically until the top surface of the plate strikes the bottom surface of the next higher shelf. Continued inflation of the bladder forces the stoppers into the necked-down portion of the vials and results in a sealing of the vials.
Prior art shelf arrangements have experienced various problems such as high frictional forces due to misalignment of the sliding surfaces thereof which have restricted proper operation of the prior art shelf arrangements. Those prior art arrangements utilizing multiple bladders are expensive in relation to the usable freeze drying volume within the drying chamber.